Conversion of electrical power from an input AC form to an output AC form is extensively used in industrial applications. One example of the applications is the aerospace industry in which electrical power is utilized to operate aircraft subsystems. Most of the aircraft subsystems are driven by variable speed motors that are controlled by varying motor input frequency and voltage. To drive these motors at a predetermined speed and/or torque, an aircraft AC power supply, which is typically 115/220 V (three-phase 400-800 Hz), is converted into an motor input power with variable frequency and voltage.
In several applications, three-phase bidirectional converters have been employed. Conventional converters include three-phase PWM inverters coupled to a DC-link that provides reactance, such as a capacitor or inductor, for back-to-back voltage or current source configurations. Such converters include two stages for conversion, the first of which converts three-phase AC to DC, and the second of which converts DC to three-phase output AC having variable voltage and/or frequency. For example, the converter includes a transformer rectifier unit (TRU) and an inverter with a capacitor interposed at a therebetween. The TRU operates to convert an aircraft AC power provided by a power source into DC power and eliminates the current harmonics created by the rectification of the AC to DC power. The inverter operates to converts the DC power into AC input power with variable frequency and voltage. The DC-link is impressed by the capacitor, which can decouple both the TRU and the inverter for control purposes.
However, the conventional converters could stand improvement with regard to meeting the demand for high power density by the aircraft applications because the two-stage conversions do not have sufficient energy efficiency, and because transformer rectifier units and the DC-line energy storage elements, such as a capacitor or inductor, are heavy and have a large physical volume and reduces a system lifetime. Thus, there is a continuing need for improved power converter topologies that provide high efficiency in converting input AC power to output AC power and achieve high power density that is required in aircraft applications.